The fabrication of integrated circuits is a complex and expensive process which requires extremely tight tolerances and has little margin for error. A particular integrated circuit, such as a microprocessor, is fabricated according to a set of recipes which specifies the materials and processing steps which are necessary to result in a finished working product in accordance with the design specification. Typically, integrated circuits are fabricated in batch on wafers, wherein multiple copies of the particular integrated circuit are fabricated on a single wafer and later separated. Multiple wafers may be processed in parallel to achieve a desired manufacturing volume.
Each of the recipes further details the parameters of a particular process step, e.g. what processing machine as well as what control parameters for the particular processing machine should be used. Processing machines include chemical vapor deposition (“CVP”) devices, chemical-mechanical-planarization (“CMP”) devices, etching devices, such as wet etching or plasma etching devices, optical or electron beam, a.k.a., e-beam, imaging devices, such as scanning electron microscopes, etc. These processing machines perform their particular process on one or more wafers subject to a myriad to control parameters. Often, the same process/processing machine is repeatedly used using different recipes so as to fabricate the different parts/layers of the integrated circuits, all according to the overall design specification. For example, a typical integrated circuit will be processed through several different CVP, CMP, lithography and etch processes, each according to a particular recipe, to build up the many layers of transistors and interconnections which make up the integrated circuit. Control parameters, as provided in the recipe, for these processing machines, which may vary among different products, different processing machines and/or different processing stages, include, but are not limited to, duration of processing, processing rate, temperature, pressure, composition of processing materials/chemicals, and other variables.
In a manufacturing environment, a particular recipe will be performed over and over on batches of wafers, referred to as lots, with each processing machine being used to repeatedly perform the same process with the same or different control parameters, depending on the particular stage of production or, as will be described below, on the results of quality control inspections/measurements. In addition, the manufacturing facility may provide multiples of particular processing machines so as to allow the parallel processing of batches of wafers and boost production capacity. Further, the particular manufacturing facility may be used to produce many different types of integrated circuits, according to different recipes, i.e. using the same processing machines with the same or different control parameters. In actual production, some production runs of a given integrated circuit may require using more of the available manufacturing capacity than other runs, i.e. using more of the available processing machines to run processes in parallel if necessary. The amount of the available capacity which may be used may be dependent upon the production goals for the particular product during a particular time period and may vary from day to day. Further, production may be affected by the unavailability of processing machines, such as unavailability due to required maintenance or repair, expected or unexpected, the return of processing machines back into service, or other manufacturing events.
The control of workflow through the manufacturing process is referred to as process control. Process control refers to the overall concept of controlling the manufacturing processes, i.e. the implementation of a given set of recipes, to end up with functional products within the specified tolerances, i.e. functioning integrated circuits. The wafer fabrication process requires a high degree of precision, where one error can compromise an entire production, necessitating a high degree of process control.
Essentially, the object of process control is to repeatedly produce a product that falls within its design and operational specifications at a profitable rate. Factors which may affect the results of a given process include: environmental factors, such as the ambient temperature or humidity; process machine factors, such as calibration, operating temperature and wear; and materials factors, such as the composition of the product undergoing processing or the composition of process consumables. These factors may vary over time as well as between similar processing machines and similar recipes, necessitating continued evaluation and refinement of the control parameters of the process, i.e. refinement of the recipe, to compensate.
Process control is accomplished by enacting quality control methodologies in the production process to monitor the quality of production and detect problems quickly so that they may be resolved quickly. In cases where in situ measurements are not possible, it is necessary to evaluate the finished product to determine if the specified tolerances are met. For example, after each processing step, measurements or testing may be performed on a sample wafer, either an actual production wafer or a test wafer included in the production run, to determine if the processed product is within the specified tolerances. Alternatively, the particular process may be tested on a test wafer/batch before being used in actual production. Where the resultant product is not within the specified tolerances or the product is within the specified tolerances but testing indicates that the process is drifting toward being out of tolerance, measures can be taken to correct the situation, such as by adjusting the control parameters of the particular processing machine, i.e. modifying the recipe. This is also referred to as Run-to-Run control, wherein the production results of a particular processing machine are continuously measured/evaluated. These measurements/evaluations are then utilized to adjust the recipe, i.e. the processing parameters of the processing machine, prior to the next run through that processing machine to compensate for any detected errors. This process is iteratively repeated such that the processing parameters of a given recipe on a given processing machine are continuously evaluated, adjusted and refined, effectively evolving with each successive generation of product, ultimately to a relatively stable state. Given the variability in the factors that can affect the results of a given process, it is likely to have two similar processing machines performing the same process but with different recipes to achieve results according to the design specification.
Statistical process control refers to the use of sampling and statistical computations to analyze the measured quality control data. This analysis can then be used to more accurately modify the recipe. Further this analysis may be used to predict and correct trends in the process results, such as a trend that the process is tending to deviate outside of the specified tolerances, though it has not yet done so.
General process control techniques, including statistical process control techniques, work well for high volume production runs, or production runs having a relatively stable throughput, because the large number of production runs provide many opportunities to test and refine the various process control stages and the consistent use of the various processing machines acts to keep those processing machines properly calibrated and their control parameters in a consistent/relatively stable state of refinement. Further, the time/resource cost of running test production runs to ensure accurate processing is minimal compared to the time/resource costs devoted to actual production. However, such techniques do not work well for low volume production runs or production runs where the production volume may vary from day to day. In such situations, there may not be enough actual production runs to generate statistically significant quality control data so as to be able to refine the various process control stages and further, the use of test production runs, which likely result in unusable products, may not be justifiable based on the cost/resources consumed as compared to the volume of actual production. In addition, for varying production volumes, excess production capacity on any one day may result in idle process machines for that day, while on a day with more significant demand, those idle process machines will be put into operation, necessitating re-calibration and testing of recipes to ensure production tolerances are met on the formerly idle processing machines being pressed into service. Similarly, processing machines may be idled by the need for maintenance, repair or due to some other event, again necessitating re-calibration and testing of recipes to ensure production tolerances are met before they are put into production.
Accordingly, there is a need for a method and system of process control which can be used efficiently in low volume production, variable volume production, as well as in situations where an unanticipated manufacturing event occurs.